1. Field of the Invention
The present invention relates to a swirling-type melting furnace for gasifying various combustible wastes and/or coal, and a method for gasifying wastes by such a swirling-type melting furnace, and more particularly to a method for treating wastes to achieve thermal recycling, material recycling, and chemical recycling.
2. Description of Related
It has heretofore been customary to treat a considerable amount of wastes such as municipal wastes, waste tires, sewage sludges, and industrial sludges with dedicated incinerators. Night soil and highly concentrated wastes have also been treated with dedicated wastewater treatment facilities. However, large quantities of industrial wastes are still being discarded, thus causing environmental pollution and shortage of landfill sites. There has been a demand for practical use of gasification and slagging combustion systems in which wastes are gasified at a low temperature and then the generated gases are combusted at a high temperature to convert ash content into molten slag and to decompose dioxins completely.
A certain domestic chemical company has already industrialized a technology for producing ammonia from hydrogen which has been produced by gasifying coal. According to this technology, a Texaco-type gasification furnace is used. In the Texaco-type gasification furnace, a coal-water mixture produced by pulverizing coal and mixing the pulverized coal with water is supplied together with oxygen from a downwardly directed burner to gasify the mixture in a single stage at a high temperature of 1500.degree. C. The coal is converted into the coal-water mixture which is of a concentration of about 65% coal, and hence can be gasified stably under a high pressure of 40 atm. The Texaco-type gasification furnace is also used in demonstration plants for combined-cycle power generation systems in the U.S.A. Examples are the Cool Water project at Daggett in California and the Tampa power project at Tampa in Florida.
FIG. 15 of the accompanying drawings shows a coal gasification process employed in the Cool Water project. As shown in FIG. 15, the system for performing the coal gasification process includes a Texaco-type waste-heat-boiler-type gasification furnace 100 having a combustion chamber 106, a slag separation chamber 107, a radiation boiler 108, and a water tank 109. The system further includes a lock hopper 110, a reservoir 111, a screen 112, a convection boiler 113, a scrubber 114, and a reservoir 115. The symbols a, c, d, and g represent a highly concentrated coal-water mixture, oxygen, steam, and slag granules (composed of coarse slag granules g.sub.c and fine slag particulates g.sub.f) respectively. Further, the symbols h, i, and j represent generated gas, water, and residual carbon, respectively.
FIG. 16 of the accompanying drawings shows a direct-quench-type gasification furnace as another Texaco-type gasification furnace. In FIG. 16, the direct-quench-type gasification furnace has a burner 101, a throat 102, a guide tube pipe 103, a gas outlet 104, a slag separation chamber 107, a combustion chamber 106, a water tank 109, a slag outlet 116, and a cooling water pipe 117. The symbols a, c, g, and h represent a highly concentrated coal-water mixture, oxygen, slag granules, and generated gas, respectively. Further, the symbols k, m, n, o, and p represent make-up water, wastewater, slag mists, slag layer, and slag droplets, respectively.
The highly concentrated coal-water mixture a is blown together with the oxygen (O.sub.2) c from the burner 101 on the top of the furnace into the combustion chamber 106. In chamber, the highly concentrated coal-water mixture a is gasified at a high temperature under a high pressure to generate gas composed mainly of hydrogen (H.sub.2), carbon monoxide (CO), carbon dioxide (CO.sub.2) and steam (H.sub.2 O). Ash content in the coal is melted at the high temperature and converted into the slag mists n which are mostly attached to the wall surface of the furnace, thus forming the slag layer o. The slag flowing down in the slag layer o passes through the throat 102, and falls as the slag droplets p into the slag separation chamber 107. The slag mists n that remain in the gas enter into the slag separation chamber 107 through the throat 102 together with the gas. In the slag separation chamber 107, the gas and the slag mists go down in the guide tube 103, and are blown into water in the water tank 109 and cooled therein. After the gas is cooled to a saturation temperature of the water under the conditions at that time, it is discharged from the gas outlet 104. The slag granules g which have been water-quenched into a glass-like material are deposited on the bottom of the water tank 109, and then discharged from the slag outlet 116. The water in the water tank 109 is discharged as the wastewater m into a discrete settler (not shown).
According to the process of gasifying wastes at a low temperature and then gasifying them at a high temperature, the high-temperature gasification furnace at the subsequent stage suffers the following problems: The gas supplied from the low-temperature gasification furnace to the high-temperature gasification furnace contains combustible gas such as hydrogen or carbon monoxide having a high combustion rate and char having a very low combustion rate. Therefore, when the gas is contacted with oxygen, the combustible gas having a high combustion rate is selectively partially combusted. Therefore, the conversion ratio of char in to gas is low.
When the gas flows in a direction opposite to gravity, since the slag flows by gravity in a direction opposite of the gas flow, the slag contained in the gas tends to be deposited on the furnace wall to such an extent as to clog the passage of the gas.
It is therefore an object of the present invention to provide a two-stage gasification system comprising a swirling-type melting furnace which is capable of treating various wastes without converting them into a cool-water mixture, having a high load capacity, and producing a relatively small amount of residual carbon.